The present invention relates to a method for cleaning synthetic resin articles and an apparatus for the method. More particularly, it relates to a cleaning method which is suitable for a treatment prior to printing, plating or coating of synthetic resin articles, producing surfaces having smoothness and no pinholes. Further, the present invention relates to a dip-coating apparatus for producing smooth coating films.
There are produced various kinds of synthetic resin articles which are either improved in appearance or changed in surface property by treatments such as printing, plating or coating. In these treatments, the cleanliness of surfaces to be treated is very important and it is not too much to say that the cleanliness exerts a decisive effect on the performances of the final products.
That is to say, the results of printing, plating and coating depend upon how to remove foreign substances from the surface of a substrate. When the foreign substances are not removed, they stay between the substrate surface and the printing, plating or coating film, and act to weaken the adhesion power between the surface and the film, or to promote the formation of pinholes or specks.
As the treatment prior to printing, plating or coating, various cleaning methods which use organic solvents, emulsion liquors, detergents, acids, or ultrasonic waves, are employed industrially. Of these methods, however, there are relatively few which are suitable for cleaning synthetic resin articles made of vinyl resins (e.g. polyvinyl chloride), acrylic resins (e.g. polymethyl methacrylate), styrene resins (e.g. polystyrene, acrylonitrile-styrene-butadiene terpolymers, acrylonitrile-styrene copolymers, methyl methacrylate-butadiene-styrene terpolymers) or polycarbonate resins. For example, in the solvent cleaning with an organic solvent such as benzene, gasoline, trichlene or perchloroethylene, it often occurs that the articles to be cleansed are dissolved in the solvent or produce cracking or blushing by contact with the solvent. Consequently, the solvent cleaning is improper for synthetic resins. The emulsion cleaning is also improper when these solvents are used, because the same problems as above occur. Further, the acid cleaning generally used for metals is also undesirable because it uses strong acids which easily do damage to the articles to be cleaned.
The detergent cleaning is relatively suitable for cleaning synthetic resin articles and it gives further desirable results when carried out with ultrasonic waves. When a detergent and ultrasonic waves are used together, dirt is removed from the surface of the articles, but the articles do not dissolve in the cleaning liquor nor produce cracking and blushing. When a detergent is used, it becomes necessary to completely remove the detergent from the surface of the articles. If the removal is insufficient, it results that adhesion between the surface and film becomes poor and surface smoothness is damaged. In addition, as the cleaning is carried out with water, drying after the cleaning is slow and dirt in the air easily adheres to the cleaned surface.
Next, it is well known that materials of relatively simple shape, for example, a flat plate, are coated by a method which comprises dipping them in a coating liquor and pulling them up slowly thereby forming a coating film on the surface. In this case, the coated surface forms a flat or relatively gentle curved surface so that, when foreign substances such as dust are present in the coating liquor, the poor smoothness of the coating film becomes readily noticeable. This becomes a serious problem when the coating film is thin. The foreign substances in the coating liquor include not only those contained in the liquor from the beginning, but also those produced by gelation or drying of a part of the liquor, those coming into the liquor from the atmosphere and those attached to the surface of articles to be coated.
In the so-called dip-coating process in which articles to be coated are dipped in a coating liquor and pulled up slowly whereby a coating film is formed thereon, it is generally the case that the articles are dipped in and pulled up from a stationary coating liquor. In this process, foreign substances in the liquor come into the coating film. In many cases, however, the smoothness of film surface is not much affected because, in general, the film thickness is relatively large, or the smoothness of film surface is not essential to the performances of the products. When the coating film is thin, the smoothness of film surface is adversely affected by foreign substances and thus its value as goods is largely damaged. The commonly used, well-known filtration process for removing dust from a coating liquor is as follows: a coating liquor is supplied to a dip-coating vessel at one side of the top (the term "top" means the continuous upper end of the surrounding wall of the vessel), allowed to flow on the surface layer of the liquor in the vessel and to overflow from the other side of the top. The liquor which overflows is filtered and circulated to the vessel. Foreign substances deposited on the bottom are discharged from the bottom. This process is suitable for preventing foreign substances in the atmosphere from entering the liquor, but once the foreign substances enter the liquor, those having a specific gravity close to that of the coating liquor and those of fine size are dispersed in the liquor without settling to the bottom. Consequently, they can not be removed.
An object of the present invention is to provide a detergent cleaning method which is free from the above-mentioned problems and at the same time suitable for printing, plating and coating of synthetic resin articles, and an apparatus therefor.
Another object of the present invention is to provide an apparatus which is capable of removing foreign substances which are difficult to remove when dispersed in a coating liquor, thereby providing a smooth coating film having very little unevenness.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
The present invention provides a method for cleaning synthetic resin articles, which comprises (1) carrying the articles into a cleaning zone where they are contacted with an aqueous detergent solution which is allowed to flow upward and to which an ultrasonic wave is applied, (2) carrying the articles from the cleaning zone to a rinsing zone where they are contacted with warm water which is allowed to flow upward, (3) carrying the articles from the rinsing zone to a dipping zone where they are contacted with more fresh warm water which is allowed to flow upward, and then (4) pulling the articles up slowly from the dipping zone to drain off the water attached to the articles. The present invention also provides an appartus for cleaning synthetic resin articles comprising a transporter capable of holding the articles to be cleaned and carrying them vertically and horizontally, an ultrasonic cleaning vessel having such a structure so as to provide an upward flow to an aqueous detergent solution, a rinsing vessel to which warm water is supplied through a filter and through which the warm water flows upward, and a dipping vessel to which warm water is supplied through a filter and through which the warm water flows upward (the dipping vessel hereinafter referred to as "draining vessel"), the cleaning vessel, rinsing vessel and draining vessel being placed in this order, and said articles to be cleaned being successively carried by the transporter from the ultrasonic cleaning vessel through the rinsing vessel to the draining vessel, and said articles being pulled up slowly from the draining vessel. Further, in a dipping apparatus for forming a coating film by carrying articles to be coated into a vessel to contact them with a coating liquor and pulling them up slowly, the present invention provides an improvement which comprises using a vessel having such a structure as to supply the coating liquor at the bottom and allow the liquor to flow upward through a perforated plate and to overflow the whole of the top, the vessel being equipped with an apparatus for circulating the coating liquor which overflows, and a filter for removing foreign substances from the liquor.
As the aqueous detergent solution, there may be used aqueous solutions containing a common detergent for cleaning, for example, anionic or nonionic surface active agents. As for the concentration of the detergent in water, a reasonable concentration is sufficient. Some detergents at times produce vigorous foaming and are poor in processability, when used in too high concentrations. When the concentration is lowered to less than a critical micelle concentration, the cleaning effect substantially decreases. In general, a preferred concentration is 0.5-10 g/liter of water. A preferred temperature of the aqueous detergent solution is about 20 to 50.degree. C., and more favorably around 40.degree. C.
The ultrasonic wave applied to the aqueous detergent solution can be generated by commonly used ultrasonic generators (e.g. frequency, 25-45 kilohertz; output, 0.5-2.0 Watt/cm.sup.2).
As to the temperature of the warm water for rinsing off the detergents, the higher it is the more effective it is. But, the upper limit of the temperature is limited because synthetic resin articles are deformed by heat when the temperature is too high. The upper limit is determined by the thermal properties of the articles so that it depends upon the kind of synthetic resin and the processing condition. In general, however, one standard for determining the upper limit is a temperature of 10.degree. C. lower than the heat distortion temperature of the articles, which is measured according to the test method specified by ASTM (American Society for Testing and Materials) D-648. A preferred temperature of the warm water for rinsing off detergents on synthetic resin articles molded by the usual conditions is within a range of 40 to 70.degree. C. But, a temperature of 50 to 60.degree. C. is more desirable to sufficiently remove the detergents with no deformation of the articles. When this warm water contains a substantial amount of dusts, the dusts adhere to the surface of the articles, and enter the warm water for the draining, so that they remain on the articles after drying. Consequently, warm water for the rinsing is, if necessary, filtered before it is supplied to the vessel.
As for the filter, commonly used filters are sufficient. A preferred filter is one of fine mesh, but filters having a mesh of less than 1.mu. in size are not desirable for practical purposes, because they are expensive and high in resistance to filtration, which leads to the enlargement of equipment used and an increase in equipment cost. On the other hand, the size of dust removed from synthetic resin articles depends upon the kind of treatments such as printing, plating and coating, and the thickness of coating film. In general, however, dust of more than 5.mu. in size should be removed. Consequently, filters having a mesh of 1 to 5.mu. on the average are preferred for filtering warm water for the rinsing process. Further, as to the quality of the warm water for rinsing off the detergents, water of such a quality that passes the quality standard of city water (authorized by Ministry of Health and Welfare) is sufficient. But, when superior surface smoothness and adhesion properties are particularly required, it is desirable to further lower the contents of the organic substance, chlorine and metal in water. Water having a required quality can be obtained using a sand filter, active carbon filter, ion-exchange filter, iron-removing equipment, manganese-removing equipment or combination thereof.
Additional fresh warm water used for dipping the articles before the water attached on the articles is drained off is used for removing a trace amount of detergent which may possibly remain on the surface of synthetic resin articles to be cleaned, and at the same time for preventing the attachment of dust to the surface and drying the surface rapidly by pulling up the articles slowly out of the fresh warm water. A higher temperature is therefore desirable for increasing the rate of drying, but it must not deform the articles as in the case of the warm water used for the rinsing process. One standard to determine the upper limit of the temperature is also a temperature 10.degree. C. lower than the heat distortion temperature of the articles. When the heat distortion temperature of the articles is less than 70.degree. C., the temperature of the water should be less than 60.degree. C. In case of synthetic resin articles molded by the usual conditions, the temperature of the water is 60 to 70.degree. C. The size of dust contained in this warm water is decreased to the same degree as, or less than, that of dust in the warm water for the rinsing of detergents. For the filters, those having a mesh of 1 to 5.mu., preferably 1 to 3.mu., in size are suitable for use. The quality of the warm water is of the same degree as or higher than that of the warm water for rinsing. Further, it is preferred that the articles are pulled up slowly out of the warm water, because water drops which remain on the surface of the articles are reduced and a uniform surface is easily obtained. However, the rate of pulling-up is increased with an increase in the temperature of the warm water. When the temperature is 60 to 70.degree. C., the rate is 20-30 cm/min.
The articles cleaned by the method of the present invention give good results when they are dried and subjected to treatments such as printing, plating and coating. The coating can effectively be carried out using the present dipping apparatus which is hereinafter explained and is another object of the present invention. Needless to say, the articles cleaned by a known method other than the present method can be coated using the dipping apparatus of the present invention.